1. Field of the Invention
This invention relates generally to kitchen ventilation systems, and more particularly to an exhaust hood and associated ductwork for handling cooking fumes generated during cooking with different types of cooking appliances situated under the hood.
2. Description of the Prior Art
Various states in the United States have very specific guidelines for the food service ventilation industry. Perhaps the most stringent guidelines have been established by the State of Michigan. Among other aspects of the guidelines are four categories of ventilation rates for different types of cooking appliances as follows:
1. Low heat, low grease; PA1 2. Grease producing; PA1 3. Heat and grease producing; PA1 4. High heat and grease producing.
In various food service establishments, it is the norm that appliances from one, more than one, or even all categories will be found in a line under the same exhaust hood. Therefore, different ventilation rates are needed at different locations along the length of the hood in accordance with the type of appliances in the line-up under the hood.
The State of Michigan as well as National Building Codes including BOCA (Building Officials and Code Administration) ICBO (International Conference of Building Officials) and Fire Prevention Codes NFPA (National Fire Prevention Association), author codes regulating the manufacture of commercial grease hoods, and all codes mentioned require that baffle type grease filters, tested in accordance with Underwriters Laboratories standards, be used. These filters remove from the ventilated air stream, grease vaporized and emitted during the cooking process, and provide a fire barrier between the portion of the hood exposed over the cooking appliance, and the concealed portion of the hood located behind the grease filters. Overlapping baffles, within each filter, create a barrier to prevent the flames generated during a cooking appliance grease fire, from passing directly through unobstructed openings into the ventilation duct which runs through concealed building spaces above ceilings and into other rooms. The overlapping baffles require the air passing through the filter to change directions to maneuver around the baffles. These changes in directions or turning of the air stream creates centrifugal forces at each turn. This centrifugal force causes the heavier grease vapor and grease droplets to be thrown against the opposing baffle. The grease is collected on, or attaches, to the baffle face and drains down the baffle to a drip trough, located beneath the filter. The trough is pitched for drainage of the collected grease to a removable grease catch pan.
State of Michigan ventilation requirements require each filter manufacturer to publish a chart indicating optimum performance. These charts indicate, in accordance with manufacturer's testing, the optimum velocity range for peak filter efficiency. Codes as well as good ventilation practice require the filter be sized to the cubic feet per minute (cfm) being ventilated. In addition, the guidelines and good practice require the filters to run the full length of the ventilator exhaust chamber. When filters do not fit the exact length of the exhaust chamber, the Michigan guideline limits the space utilizing blank panels or filler panels, to complete the shortage in filter length, to 16% of total filter area. Virtually all baffle filters require the exhaust air to make a minimum of two turns around the baffles as it passes through the filter from the intake or inlet face of the filter to the exhaust or outlet face. The State of Michigan also requires filter manufacturers to publish the velocities to be maintained that will produce maximum grease extraction efficiency. This published report must also include the net free area of each filter. The net free area is used to determine which filter size will provide the required velocity at a given ventilation air volume. Industry testing of various filters have demonstrated these velocities, producing maximum grease extraction efficiency, to be between 275 feet per minute (f.p.m.) minimum and 400 f.p.m. maximum.
An additional Michigan requirement is the pressure drop across the filters be equal to or greater than 0.30" w.c. (water column) when using a single duct collar in a hood length exceeding 10'-0". Lower pressure drops can be accommodated by adding additional duct take offs. These additional take offs add installation and maintenance expenses. As the ducts are above the hood and therefore normally concealed above the ceiling there is often insufficient space to accommodate the additional duct. The connecting duct transport velocity is restricted by code to a minimum of 1,500 f.p.m. and a normal maximum of 4,000 f.p.m. As a result of the combination of requirements of the codes, it is very difficult to achieve ventilation in a given hood where the cooking appliances under the hood are in different categories. It is even more difficult to obtain optimum ventilation performance and/or economy. In addition, if the requirements of the food service establishment indicate a need for a different arrangement of different categories of cooking units under the hood, it can be very difficult to achieve the code-specified filter performance, much less optimize the arrangement. Also, if different filters are used, they are not likely to fit into the same grease filter frame, thus requiring different hood designs. Therefore, if appliances are changed or relocated under the hood, and more or less air is required, a new hood will be required. Changes going from low heat and low grease, to heat and grease, can be impossible because the smaller filters for the low heat and low grease application creates such increased pressure drop that the fan cannot provide the required volume of air flow for the larger filters.
As mentioned above, one of the Michigan requirements pertains to a minimum pressure drop requirement when using a single duct collar in a hood length exceeding ten feet. Lower pressure drops can be accommodated by adding additional duct take-offs, but these additional take-offs add installation and maintenance expense. Nevertheless, it is sometimes necessary. In addition to state codes, installations must meet the requirements of local government codes. There is a trend for governmental municipalities to accept only hoods bearing the label of one of the nationally recognized testing laboratories. Listed hoods bearing such labels have specifications for placement of an exhaust duct opening collar within a limited range of locations relative to the transverse center line of the hood. But problems may arise at the hood installation site, because building codes prohibit any installation such that roof joists, plumbing lines, electrical lines or other devices would penetrate the exhaust duct running from the hood to the roof-mounted fan. However, such building components typically run in the space between ceiling above the hood, and the roof. Therefore it is frequently a problem to coordinate the factory installed location of the exhaust collar with the field location of obstructions. Consequently, it is common to see in the field, a labeled hood which has been installed with the factory located collar cutout welded closed and the duct welded to the hood in a different location that was necessitated by structural members or mechanical services already in the building and which could not be moved. Such field welding of a collar is not in compliance with the testing laboratory listing, and can put the burden of acceptance or rejection on a local government code official who does not want that burden or is not trained to make the necessary judgment.
In addition to the above-mentioned hood-to-duct connection problem resulting from building mechanicals being in the ceiling space, the routing of the duct to the roof-mounted exhaust fan can also be a challenge. It sometimes involves turns in direction or long duct runs. Codes require that a clean-out or access door be provided at each turn in direction and at intervals no greater than twenty feet apart in long duct runs. Typically the access doors include frames welded to the perimeter of an opening cut into the wall of the duct. Then a door is screwed or otherwise fastened to the frame. The welding is difficult to accomplish, since the space available in which to work is very limited. Yet it is desirable to install the clean-outs after the ducts are in place in the building, to be certain that there is no conflict with other mechanicals that would restrict access to them for clean-out. Since it is necessary to route ductwork in the field, joints are inevitable. Building codes require not only that the duct sections themselves be constructed with liquid-tight continuous external welds at all seams and joints, but also that all joints made in the field during installation must be made using the same liquid-tight continuous external welds. As indicated above, these code requirements make kitchen ventilation system installations difficult and expensive.
Some efforts to address at least the problem of different requirements for different types of cooking units have been made and the results disclosed in some United States patents. For example, the U.S. Pat. No. 4,281,635 issued Aug. 4, 1981 to Gaylord, addresses the problem of different amounts of air pollution by different cooking units, but does so in a water-wash hood. Such hoods typically cost two to three times as much as canopy hoods with dry filters. The Gaylord patent teaches the use of choke attachments such as 60, 61 and 63, cut to a length corresponding to the combined width of the low polluting cooking units with which a portion of the hood length is associated. They are installed by spot welds, screws or "other suitable means". They are used to throttle, choke or otherwise restrict the inlet flow to that portion of the ventilator in which they are used. They can be removed by removing the screws and burning off the spot welds, for example.
Dry filters of the baffle type can be made along the lines shown generally in U.S. Pat. No. 3,870,494 issued Mar. 11, 1975 to Doane. An approach to providing different capacities in different portions of a hood using baffle filters is to use a filter unit which is adjustable in itself. An example is shown in U.S. Pat. No. 3,566,585 issued Mar. 2, 1971 to Voloshen et al. Such filters have an adjustment screw 50 operable to change the spacing between baffle members. But such filters are expensive, and the adjustment feature is either not used or not effectively usable. A McCauley U.S. Pat. No. 4,346,692 issued Aug. 31, 1982 uses adjustable louvers or damper blades in the hood itself or in a make-up air module, but not tailored to accommodate individual requirements of a variety of types of cooking equipment under the hood. Similarly, the Neitzel et al. U.S. Pat. No. 4,373,509 issued Feb. 15, 1983, provides adjustable dampers in the hood to change relative amounts of fresh air and tempered air used as make-up air in a kitchen ventilating system as various combinations of cooking equipment are used.
The present invention is addressed to facilitating original installation of a kitchen ventilation system, and minimizing the necessity of any changes of hood or duct to accommodate changes in cooking equipment types and combinations of types served by the exhaust hood in the system.